Deriving debris‐flow dynamics from real‐time impact‐force measurements

Understanding the impact forces exerted by debris flows is limited by a lack of direct field measurements and validated numerical models. In this study, we use real-time impact-force measurements and field observations of debris flows recorded by a sensor network in Jiangjia Ravine, China, to quantify the impact-force distribution of natural debris flows. We observed one debris flow event during and after a storm on August 25, 2004, including 42 short-duration surges and seven long-duration surges, and impact-force signals were successfully recorded for 38 surges. Our observed debris flows comprise high-viscosity laminar flows with high sediment concentration and frequent solid-to-solid interactions. We identified a large magnitude (up to 1 kN), high-frequency (greater than 1 Hz) fluctuating component of the impact force that we interpret as solid particle impact on the sensors. The variability of particle impact forces increases with the mean impact force. Our results show that a log-logistic distribution can describe the probability density distribution of impact forces. Solid-dominated surges and fluid-dominated intersurge flows have similar impact-force distributions, but surges usually have heavy tails. We created a dimensionless number to describe the impact force and correlated it against existing dimensionless parameters. Finally, we develop a simple particle impact model to understand the relationship between flow dynamics and the impact force inside debris flows that could be applied to improve debris-flow flume experiments and design debris-flow hazard mitigation measures.